In recent years, there is a tendency that the driving form of an automobile has shifted from a conventional driving form, using an internal combustion engine, to a driving form using an electric motor. Under the circumstances, it has been noticed to use, as the wheel bearing apparatus for an electric vehicle, a wheel bearing apparatus incorporating an in-wheel motor with a combination of a wheel bearing, a reduction gear and an electric motor. The use of the wheel bearing apparatus incorporating an in-wheel motor for a driving wheel of an electric vehicle enables a remarkable reduction in weight and size of the vehicle. This is due to the elimination of bulky and complicated power transmitting mechanisms comprised of conventional parts such as a propeller shaft, a differential gear etc.
One example of a wheel bearing apparatus incorporating an in-wheel motor is shown in FIG. 8. The wheel bearing apparatus incorporating an in-wheel motor includes a wheel bearing 51, a planetary reduction gear 52 mounted on the wheel bearing 51, an electric motor M for driving the planetary reduction gear 52, and a driving portion 54 having a rotation member 53.
The wheel bearing 51 includes a wheel hub 56 having, on its one end, a wheel mounting flange 55 for mounting a wheel. The wheel hub outer circumference includes one inner raceway surface 56a. A cylindrical portion 56b axially extends from the inner raceway surface 56a. An inner ring 57 is press-fit onto the cylindrical portion 56b of the wheel hub 56. The inner ring outer circumference includes the other inner raceway surface 57a. An outer member 58 has an integrally formed body mounting flange 58b on its outer circumference. The body mounting flange 58b secures the outer member 58. The outer member inner circumference includes double row outer raceway surfaces 58a, 58a that oppose the inner raceway surfaces 56a, 57a. Double row rolling elements 59, 59 are freely rollably contained between the outer and inner raceway surfaces.
The planetary reduction gear 52 includes a sun gear 60 arranged on a rotation member 53, internal teeth 58c formed on the inner circumference of the outer member 58, multiple planetary gears 61 mating with the internal teeth 58c and the external teeth 60a of the sun gear 60, and carrier pins 63 that rotationally support the planetary gears 61 relative to the connecting shaft 62.
The driving portion 54 includes a cup-shaped stator housing 64 forming the electric motor M and separably arranged on the outer member 58. A stator portion 65 is built in the stator housing 64. A rotor portion 66 is secured on the rotation member 53 and is arranged opposite to the stator portion 65 via a predetermined air gap. The connecting shaft 62 is connected to the wheel hub 56, via serrations 62a, to transmit the rotation of the electric motor M to the wheel hub 56, via the planetary reduction gear 52 and the connecting shaft 62 so as to drive the wheel.
Such a prior art structure of the wheel bearing apparatus is able to keep a sufficient space for the wheel bearing 51. Also, it enables easy separation of the wheel bearing 51, the planetary reduction gear 52 and the driving portion 54 from each other in order to replace them. This makes it unnecessary to replace the whole unit for service. Thus, this reduces both the resource and the maintenance cost. (See e.g., Japanese Laid-open Patent Publication No. 231564/2005).
However, the prior art wheel bearing apparatus has the following problems. Since the sealability between the wheel hub 56 and the connecting shaft 62 is imperfect, lubricant contained within the driving portion 54 tends to leak to the outside. Accordingly, the leaked lubricant may affect the ambient environment or adhere to adjacent brake parts.